Fiber optic networks are becoming increasingly popular for data transmission due to their high speed, high capacity capabilities. Multiple wavelengths may be transmitted along the same optic fiber. These wavelengths are combined to provide a single transmitted signal. A crucial feature of a fiber optic network is the separation of the optical signal into its component wavelengths, or "channels", typically by a dense wavelength division multiplexer. This separation must occur in order for the exchange of wavelengths between signals on "loops" within networks to occur. The exchange occurs at connector points, or points where two or more loops intersect for the purpose of exchanging wavelengths.
Add/drop systems exist at the connector points for the management of the channel exchanges. The exchanging of data signals involves the exchanging of matching wavelengths from two different loops within an optical network. In other words, each signal drops a channel to the other loop while simultaneously adding the matching channel from the other loop.
FIG. 1 illustrates a simplified optical network 100. A fiber optic network 100 could comprise a main loop 150 which connects primary locations, such as San Francisco and New York. In-between the primary locations is a local loop 110 which connect with loop 150 at connector point 140. Thus, if local loop 110 is Sacramento, wavelengths at San Francisco are multiplexed into an optical signal which will travel from San Francisco, add and drop channels with Sacramento's signal at connector point 140, and the new signal will travel forward to New York. Within loop 110, optical signals would be transmitted to various locations within its loop, servicing the Sacramento area. Local receivers (not shown) would reside at various points within the local loop 110 to convert the optical signals into the electrical signals in the appropriate protocol format.
The separation of an optical signal into its component channels are typically performed by a dense wavelength division multiplexer. FIG. 2 illustrates add/drop systems 200 and 210 with dense wavelength division multiplexers 220 and 230. An optical signal from Loop 110 (.lambda..sub.1 -.lambda..sub.n) enters its add/drop system 200 at node A (240). The signal is separated into its component channels by the dense wavelength division multiplexer 220. Each channel is then outputted to its own path 250-1 through 250-n. For example, .lambda..sub.1 would travel along path 250-1, .lambda..sub.2 would travel along path 250-2, etc. In the same manner, the signal from Loop 150 (.lambda..sub.1 '-.lambda..sub.n ') enters its add/drop system 210 via node C (270). The signal is separated into its component channels by the wavelength division multiplexer 230. Each channel is then outputted via its own path 280-1 through 280-n. For example, .lambda..sub.1 ' would travel along path 280-1, .lambda..sub.2 ' would travel along path 280-2, etc.
In the performance of an add/drop function, for example, .lambda..sub.1 is transferred to path 280-1. It is combined with the others of Loop 150's channels into a single new optical signal by the dense wavelength division multiplexer 230. The new signal is then returned to Loop 150 via node D (290). At the same time, .lambda..sub.1 ' is transferred to path 250-1 from 280-1. It is combined with the others of Loop 110's channels into a single optical signal by the dense wavelength division multiplexer 220. This new signal is then returned to Loop 110 via node B (260). In this manner, from Loop 110's point of view, channel .lambda..sub.1 of its own signal is dropped to Loop 150 while channel .lambda..sub.1 ' of the signal from Loop 150 is added to form part of its new signal. The opposite is true from Loop 150's point of view. This is the add/drop function.
Conventional methods used by dense wavelength division multiplexers in separating an optical signal into its component channels includes the use of filters and fiber gratings as separators. A "separator," as the term is used in this specification, is a unit of optical components which separates one or more channels from an optical signal. Filters allow a target channel to pass through while redirecting all other channels. Fiber gratings target a channel to be reflected while all other channels pass through. Both filters and fiber gratings are well known in the art and will not be discussed in further detail here. FIG. 3 illustrates a conventional multi-stage serial cascade configuration of separators in a dense wavelength division multiplexer 300. In this conventional method, each separator targets only one channel to be filtered/reflected and sent along a path. For example, an optical signal containing channels .lambda..sub.1 -.lambda..sub.n is inputted into separator 310A, which filters/reflects channel .lambda..sub.1 and send it along its own path 320-1. The remaining channels .lambda..sub.2 -.lambda..sub.n are sent to the next separator 310B, which filters/reflects channel .lambda..sub.2 and sends it along its own path 320-2. This continues until each channel has been filtered/reflected and sent along its own path. Thus, with this method, for N channels there are N separators.
FIG. 4 illustrates a conventional single stage parallel configuration of separators in a dense wavelength division multiplexer 400. In this conventional method, the original optical signal containing .lambda..sub.1 -.lambda..sub.n enters a signal splitter 410 which splits the signal onto N separate paths, each split signal containing channels .lambda..sub.1 -.lambda..sub.n. Each of these split signals is sent along a separate path 420-1 through 420-n. Each signal is then filtered or reflected by the separators 430A-430N to output one particular channel. For example, a split signal containing channels .lambda..sub.1 -.lambda..sub.n exits the splitter 410 onto path 420-1. The split signal enters separator 430A which filters/reflects channel .lambda..sub.1 and sends it along path 420-1. Another split signal containing .lambda..sub.1 -.lambda..sub.n exits splitter 410 onto path 420-2 and enters separator 430B. Separator 430B filters/reflects channel .lambda..sub.2 and sends it along path 420-2. This process repeats to separate each channel. Thus for N channels, there must be N separators plus a signal splitter.
A problem with the conventional configurations of separators above is the resulting high insertion loss. Insertion loss is the attenuation of an optical signal caused by the insertion of an optical component, such as a connector, coupler, or filter. For the multi-stage serial cascade configuration illustrated in FIG. 3, each time the optical signal goes through a separator 310A-310N an amount of insertion loss results. For example, if the optical signal in FIG. 3 has eight channels .lambda..sub.1 -.lambda..sub.8 and each component causes 1 dB of insertion loss. By the time .lambda..sub.8 is separated, it would have passed through eight separators. .lambda..sub.8 would thus suffer 8 dB of insertion loss.
The same problem exists for the single stage parallel configuration in FIG. 4. Assume again that the optical signal contains eight channels and each component causes 1 dB of insertion loss. In splitting one signal onto eight paths, a 9 dB insert loss results. Another 1 dB of loss is added by the separator 430A-430N. Thus, each channel suffers 10 dB of insertion loss.
Therefore, there exists a need for a dense wavelength division multiplexer with a method of separation which lowers insertion loss. The present invention addresses such a need.